1. Field of the Invention
The present invention is directed to scanning probe microscopes (SPMs), and, more particularly, relates to a SPM that can acquire high-quality images at high acquisition rates and to a method of operating such an SPM.
2. Description of Related Art
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a tip and low tip-sample interaction forces to characterize the surface of a sample down to atomic dimensions. Generally, SPMs include a probe having a tip that is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, characteristic data can be acquired over a particular region of the sample and a corresponding map of the sample can be generated.
The atomic force microscope (AFM) is a very popular type of SPM. A typical AFM is shown schematically in FIG. 1. AFM 10 employs a probe 12 having a cantilever 14 and a tip 16. Scanner 24 generates relative motion between the probe 12 and a sample 22 while the probe-sample interaction is monitored. In this way, images or other measurements of the sample can be obtained. Scanner 24 typically includes one or more actuators that usually generate motion in three orthogonal directions (XYZ). Scanner 24 may be a single integrated unit such as a piezoelectric tube actuator that moves either the sample or the probe in all three axes. It moves the probe 12 in the illustrated example. Alternatively, the scanner may be an assembly of multiple separate actuators. Some AFMs separate the scanner into multiple components, for example an xy actuator that moves the sample and a separate z-actuator that moves the probe. Probe 12 is often coupled to an oscillating actuator or drive 15 that is used to drive probe 12 at or near a resonant frequency of cantilever 14. Alternative arrangements measure the deflection, torsion, or other motion of cantilever 14. Probe 12 is often formed from a microfabricated cantilever 14 with an integrated tip 16.
If the AFM is configured for an oscillation mode of operation, an electronic signal is applied from an AC signal source 18 under control of a probe-sample interaction is typically controlled via force control feedback by controller 20. Motion of the cantilever 14 is monitored by directing a sensing light beam from a sensing light source (not shown), such as a laser, to the backside of cantilever 14. The beam is then reflected towards a detector 26, such as a four quadrant photodetector. As the beam translates across detector 26, appropriate signals are transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 14.
Controller 20 generates control signals to maintain either a relative constant interaction between the tip 16 and sample or a constant deflection of the cantilever 14. Measurement involves controlling the scanner 24 to move either the sample or the probe (the probe 12 in the present example) up and down relatively perpendicular to the surface of the sample 22 under feedback. The scanner 24 is controlled to perform a scan operation by effecting relative probe-sample motion in an “x-y” plane that is at least generally parallel to the surface of the sample 22. (Note that many samples have roughness, curvature and tilt that deviate from a flat plane, hence the use of the term “generally parallel.” The term “parallel” may also be used herein and should be construed to also mean “generally parallel.”) The scan typically takes the form of a raster scan in which data is taken along lines in the x direction that are closely spaced in the y direction. The maximum length of the lines in the x direction is known as the “scan range.” In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography.
The measured characteristic of probe-sample interaction will depend in part on the AFM's intended mode of operation. That is, AFMs may be designed to operate in a variety of modes including contact mode and oscillating mode. In contact mode, the probe 12 is lowered into interaction with the sample 22, and cantilever deflection or a related characteristic is monitored and controlled to a setpoint. In an oscillating mode such as the popular mode known as TappingMode™ (TappingMode™ is a trademark owned by the present assignee), the probe is oscillated by a probe oscillator 15 via an AC signal source 18 at or near a resonant frequency of the cantilever 14. A force control loop attempts to maintain the amplitude of this oscillation at a desired setpoint value to minimize the “tracking force,” i.e. the force resulting from tip/sample interaction. (Alternative feedback arrangements keep the phase and/or oscillation frequency constant or a combination of the above. As in contact mode, these feedback signals are then collected, stored and used as data to characterize the sample.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
The most broadly adopted commercial SPMs usually require a total scan time of several minutes to cover an area of several square microns at high resolution (e.g. 512×512 pixels), low tracking force, and high image quality. In general, the practical limit of SPM scan speed is a result of the maximum speed at which the SPM can be scanned while maintaining a tracking force that is low enough not to damage the tip and/or sample or to at least limit the damage to the tip and/or sample to acceptable levels.
Recent work in high-speed SPM has been performed by a number of groups, including, for example, the research groups of Paul Hansma at the University of California, Toshio Ando of Kanazawa University, Mervyn Miles at the University of Bristol, the Frenken Group at the University of Leiden, and commercial companies, such as, Olympus and Infinitesima.
Obtaining a high quality, high speed AFM image demands outstanding performance of each and every major sub-system of the AFM. Just as the strength of a chain is governed by the weakest link, the performance of a high speed AFM is often governed by its weakest or slowest subsystem. An AFM subsystem that fails to provide the necessary range, speed, bandwidth, slew rate, linearity etc. will either lead to diminished performance of the overall system and/or unacceptable flaws in the image quality. Despite some excellent progress, earlier SPM systems have not achieved the suite of simultaneous performance metrics required to broadly enable applicable high speed AFM.